The present invention relates to an acoustic board with an improved composite structure, suitable for reducing the sound produced by aircrafts with jet engines.
The nacelles assembled on aeroplanes with jet engines have absorbing linings, called acoustic boards, for reducing the sound produced by the engine itself.
These acoustic boards are designed to provide acoustic impedance to the sound produced.
In particular, the value of this impedance should be as close as possible to that necessary for obtaining maximum attenuation.
This optimum impedance value varies in relation to the sound spectrum emitted and the operating conditions of the engine.
The acoustic boards currently used usually comprise a prevalently resistive sheet made of porous material, or upper sheet, attached to a honeycomb structure, in turn attached, at the opposite side, to a sound-reflecting sheet, or lower sheet.
Other variations to the embodiment described may contain additional layers of resistive material interposed with honeycomb structures.
The upper sheets, furthermore, can be of the perforated or (acoustically) linear type.
The perforated sheets consist of layers of material having a regular perforation with holes having a specific diameter and uniformly distanced, so as to obtain the desired porosity. The acoustic resistance of this type of material shows a high sensitivity to the local sound velocity of the particles. For these boards, the local sound velocity of the particles depends on the level of acoustic pressure and on the Mach number of the air flow which brushes against the board.
The so-called xe2x80x9clinearxe2x80x9d sheets consist of layers of fabric, whose porosity produces an acoustic resistance which has a minimum variation in relation to the sound velocity.
The layer of fabric, which can be of any material, is attached to a supporting sheet which can be produced with various metallic and non-metallic materials.
It is difficult, however, to obtain an acoustic resistance which forms the best compromise for all operating conditions of the engine.
An objective of the present invention is therefore to overcome the disadvantages mentioned above and, in particular, to propose an acoustic board with an improved composite structure, which has a more efficient sound attenuation.
Another objective of the present invention is to propose an acoustic board with an improved composite structure, which allows the acoustic resistance to be adequately evaluated and controlled in relation to the sound pressure.
A further objective of the present invention is to produce an acoustic board with an improved composite structure, which maintains an adequate structural integrity also under severe functioning conditions.
These and other objectives are achieved by an acoustic board with an improved composite structure, according to claim 1, to which reference should be made for the sake of brevity.
The invention advantageously proposes to describe an acoustic board, suitable for reducing the sound of aircraft engines, having desired nominal resistance values and variability values thereof, with the sound velocity and sound pressure level.
This type of board can therefore be used to obtain an improved adaptation to the optimum characteristics defined for all functioning conditions of the engine.